The demonologist is the mightiest of sorcerers, wielding the power of hell and earth and capable of conjuring pillars of flame or titanic storms of electricity.

So great is their occult knowledge they can bargain a servant of hell into doing their bidding; a guardian daemon of warped beauty and terrible might.

Demonologists concentrate solely on sorcery and the potency of their spells is unmatched, as capable of eliminating a single enemy in a hellish inferno as destroying enemy lines with forks of lightning. They can sacrifice their own life-force in return for more occult power and they are most potent when they can devote their spell energies solely to attack.

Demonologists are magic experts who have aligned themselves strongly with otherworldly demonic forces. Their damaging magical attacks drain from the caster's own life force. The class also wields augmentative magic and can summon demonic servants to molest children.

Demonologists are a dps class. They excel at dealing high damage in a short period of time. They might not be the ones that deal the highest amount of damage, but they are very good at dealing their damage to groups of enemies. Inferno of Amher and Shockblast both have a very large area of effect. Most spells used deal at least a certain amount of area damage. Demonologists also deal damage over time, such as burning enemies, though they are not as good at it as other classes (necromancer, for example).

Demonologists get spells and feats based off of Lightning and Fire. The Fire spells/feats are a bit faster to cast, do less damage per shot and contain most of the Damage over Time (DoT) abilities, as well as some per-shot stacking damage buffs to the Demonologist for fire spells. Over time with at least most of the DoT effects active on an enemy these spells will do more damage than the Lightning ones, but this is only relevant on enemies that live long enough to take those DoT effects for long periods so this only really becomes valid on bosses. Lightning spells, on the other hand, almost completely ignore DoT effects and get all their damage out in one burst, so shot for shot they do more damage than the Fire spells. Lightning spells and feats also seem to have slightly more Crowd Control abilities in terms of roots and knockbacks.

When well played, demonologists can also act as efficient off-tanks, being able to keep an enemy off the group for a long time. Use of Storm Chains will root an enemy in place for 10 seconds. Gate of Hell will root an enemy in place, as well. Fully feated, Detonation will knock down even full-plate wearing opponent and their neighbors. Both Fiery Torment and Possession will stun the target.
Demonologists also have a couple of abilities useful for the whole group. Their pet increases life, stamina and mana regeneration, maximum life, maximum mana or damage, depending on the needs. The feat Unholy Hate grants a spell that will boost damage of the whole team for 6 seconds.
The weakness of the demonologist is the facility with which they die. Their best strategy to survive is "kill before you get killed". Use of their crowd control abilities is also highly recommended. Demonologists have a couple of last resort spells, such as Planar Shift that only comes at level 75, but greatly increases immunities and evasion rate for 6 seconds.

Still, all in all Demonologists are a bit of a "glass cannon" class, able to pump out heavy constant damage but they have very limited options for absorbing or healing damage to themselves, relying heavily on crowd control abilities to try and keep damage off of them particularly against bosses. And don't expect much damage-dealing help from your pet either like a Necromancer would as your pet's damage output is rather pathetic, barely at the level of a single one of the Necromancer's equivalent level low-grade single-point pets. You'll be relying only on its buff to you.

Demonologists make great use of the spellweaving skill. It allows them to raise their damage, while having drawbacks like enormous mana drain, resistances debuffs, self inflicted damage or continuous stamina drain.

Demonologist gains access to five buffs during spellweaving, which can be activated at each stage of spellweaving. These are listed as they appear on screen from top to bottom:

Each stage of spellweaving also brings one debuff at a time. Some of debuffs are stacking and it is unwise to stack these over three. Stacking debuffs have a 60 second timeout and the timer is refreshed with new stacks.

Spellweaving roots demonologists to the ground at all times and prevents them from turning around, but it also increases knockback resistance.

In PvP, a demonologist may not fare too well at low level, before they get all their crowd control abilities. Later, however, between range, high-damage and damage over time, demonologists may fare relatively well. Particularly against melee opponents.